To meet current emission regulations, automotive vehicles can regulate the air/fuel ratio (A/F) supplied to the vehicles' cylinders so as to achieve maximum efficiency of the vehicles' catalysts. For this purpose, it is known to control the air/fuel ratio of internal combustion engines using an exhaust gas oxygen (EGO) sensor positioned in the exhaust stream from the engine. The EGO sensor provides a feedback signal to an electronic controller that calculates A/F bias values over time. The calculated A/F bias values are used by the controller to adjust the A/F level in the cylinders to achieve optimum efficiency of the corresponding catalyst in the exhaust system.
It is also known to have systems with two EGO sensors in the exhaust stream in an effort to achieve more precise A/F control with respect to the catalyst window. Normally, a pre-catalyst EGO sensor is positioned upstream of the catalyst and a post-catalyst EGO sensor is positioned downstream of the catalyst. Finally, in connection with engines having two groups of cylinders, it is known to have a two-bank exhaust system coupled thereto where each exhaust bank has a catalyst as well as pre-catalyst and post-catalyst EGO sensors. Each of the exhaust banks corresponds to a group of cylinders in the engine. The feedback signals received from the EGO sensors are used to calculate total f/a bias values in their respective group of cylinders at any given time. The controller uses these total f/a bias values to control the amount of liquid fuel that is injected into their corresponding cylinders by the vehicle's fuel injectors.
It is also known in the art for the total f/a bias value to be comprised of two components: a short-term fuel trim value and a long-term fuel trim value. The short-term fuel trim value for a particular group of cylinders is calculated based on the feedback signals from the two EGO sensors in the corresponding exhaust bank. The short-term fuel trim value facilitates a "micro" or gradual adjustment of the A/F level in the cylinders. An example of a method used to gradually adjust the A/F level in a group of cylinders is the well-known "ramp, hold, jumpback" A/F control method described in U.S. Pat. No. 5,492,106, the disclosure of which is incorporated herein by reference. The long-term fuel trim value for a particular group of cylinders is a "learned" value corresponding to particular engine parameters and stored in a data structure for retrieval by the controller. The long-term fuel trim value is calculated based on a corresponding short-term fuel trim value and a previously-calculated long-term fuel trim value. The long-term fuel trim value facilitates "macro" A/F adjustments, which increases the A/F adjustment rate in the cylinders during times of abrupt changes in certain engine parameters, such as engine load and/or engine speed.
Sometimes, in a two-bank, four-EGO sensor exhaust system, one of the pre-catalyst EGO sensors degrades. In other circumstances, it is desirable to purposely eliminate one of the pre-catalyst EGO sensors in a two-bank system to reduce the cost of the system. In either event, it is desirable to continue to be able to adjust the A/F level in the group of cylinders coupled to the exhaust bank having only one operational EGO sensor by using both short-term and long-term fuel trim values, wherein the short-term and long-term fuel trim values are calculated from the feedback signals received from just the three operational EGO sensors alone. However, known methods for A/F adjustment require a matched set of pre-catalyst and post-catalyst EGO sensors in each bank, such as in a one-bank, two EGO sensor system or in a two-bank, four EGO-sensor system.
Accordingly, it is desirable to have a new method of adjusting the A/F level in an engine coupled to a two-bank three-EGO exhaust sensor system using both short-term and a long-term fuel trim values, both of which are calculated from the feedback signals of three EGO sensors instead of four.